Max btjchnee



Patented Feb .10, 1931 earn-1n STATES PATENT oFFIcE max BUGHNER, or HANOVER-minimum, GERMANY PROCESS OF PRODUCING ALUMINA FROM ALUMINIFEROUS SUBSTANCES No Drawing. Application filed March 7', 1928, Serial No. 259,906, and in Germany December 24, 1926.

I'ha ve filed applications for patents in Germany, December 2 1th, 1926; Germany, January 3rd, 1927 Germany, February 17th, 1927; Germany, Februaryj25th,"1927; Ger- 5 many, August 24th, 1927 Germany, September 15th, 1927 Germany, October 24th, 1927 Great "BritainyDecember 23rd, 1927; Great Britain, January 3rd,'1928; Great Britain,

January'13th, 1928; France, December 24th, 1927 Italy,'Jan uary 11th,}1928 ;"Yugoslavia,

November 9th,.1927; Yugoslavia, December 24th, 1927 Norway, December 23rd, 1927;

Austria, December 23rd,1927; Switzerland,

December 24th, 1927 Czechoslovakia, December' 31st, 1927 Czechoslovakia, January 3rd,1928; Czechoslovakim'January 9th, 1928; Hungary, December 24th, 1927 Hungary, January 3rd, .1928; Hungary, January lith, 1928. f v

Attempts have been made to prepare aluminium, oxide by thermal decomposition of aluminium salts, such as aluminium chloride, aluminium sulphate and ammonia alum. The last mentioned process was found to be economically impracticable; Aluminium sulphate must be decomposed at such a high temperaturethat the greater portion of the sulphuric acid is. decomposed into sulphur dioxide and oxygen,so'that it is only possible 0 to recover the sulphuric acid by a complicated method, or by means of expensive apparatus. The decomposition of hydrated aluminium chloride isattended with great technical difficulties and cannot be carried out in a simple, direct manner, owing to the destructive action of the hydrochloric acid.

I have found that nitric acid enables aluminiumoxide to be produced from clay and other aluminiterous substances in a very simple and economical manner.

According to my invention alumina is produced fromclay and other aluminiterous'substances by treatingthe raw material, which may be previously calcined, for example at temperatures from 550 to 800 (1, with nitric acid in vessels made of acid-proof alloys, the

resulting solution, of aluminium nitrate be ing separatedjfrom the insoluble residue, and the alumina being obtamed from the said solution by thermal decomposition. The de-' compositionof aluminium nitrate can be effected by heating the solution of aluminium nitrate'obtained or by heating the solid aluminium nitrate produced from the solution. The oxides of nitrogen evolved are used, if desired", for reforming nitric acid for treating a fresh quantity of raw material.

Examples of suitable acid-proof alloys are alloys of iron, chromium and nickel, alloys containing tungsten and the like, and refined steels. p s For decomposing the aluminiterous material it has been found advantageous to treat the damp or air-dry raw; material with 'dilute or concentrated acids. In this way 'it is possible to minimize the difficulties attending the production of pure alumina, and in particular alumina free from iron. Thus, :t'orexample, in the case 01 a bauxite containing 50% of iron oxide, 98% of the iron present can be dissolved out by simple pretreatment with concentrated hydrochloric acid. The content of aluminium is practically unaffected.

In the same manner the raw material can be treated with nitric acid. The production of the aluminium nitratesolution, that is the extraction of the alumina from the'raw material, is preferably carried out by heating the raw material with nitric acid.

According to the nature of the material to be extracted it may be necessary to prolong the heat treatment, in order 'to obtain a larger yield of alumina.

particular advantage is thepreparing of the solution of aluminium nitrate by treating an excess of aluminiferous material related as to the content of alumina with nitric acid. In spite of the deficiency of nitric acid it is surprising how much more clay passes into solution than would be assumed for the composition of normal aluminium nitrate. When working, for example, at a temperature of to 100 C. about 30 to 40% more aluminium oxide goes into so lution than corresponds to the equivalent quantity of nitric acid.

Theparticular advantage of producing basic aluminium nitrate is that, when using aluminiferous materials containing iron, the

elimination of iron occurring simultaneously is so thorough, that the aluminium oxide prepared from basic aluminium nitrate contains only about 1 to 1.5% of F6 0,. Probably this is due to the fact that aluminium oxide is dissolved in excess.

If the solution of basic aluminium nitrate,

prepared with a deficiency of nitric acid, is

allowed to stand for a few hours, or a longer period, or it itis previouslycooled, a fur ther reduction of-the iron content-of the alumina is effected to about 0.7% Fe O In some circumstances the cooling may be continued until the solution solidifies, I have. then found that aftercrystallizing out the to the elimination of iron, basic aluminium nitrate appears to occupy a unique position. The period of treatment may be considerably reduced, with saving 'of heat, .if'the aluminiferous raw material and the nitric acid are first made into a paste, and if the paste is stirred andallowed to stand for a long time at lower temperature,'to form a kind of sludge. Then the aluminium nitrate is produced by heat with the addition of further quantities of nitric'acid,

I Th i theextra on rre i p d to calcined clay, with heat and stirring, but without the pretreatment with nitric in the cold, a yield of 7 8% of alumina may be obtained, but if pre-treated in the manner dcscribedv a yield of 98.7% may result.

Impurities, for example, iron, titanium, etc., may be separated in the course of the process, For example, a high degree of purification may occur when the aluminiferous raw material is treated by nitric acid, with heat and under pressure, with or without prior-treatment of the raw material to form a sludge. Thus iron and other impurities may be separated n a surprisingly efl'ective-manner, furthermore the decomposition takes place more rapidly and there is a more complete solution of the alun'iina.

Thus, I have found when bauxite was treated for six hours with nitricacid at 8 atmospheres pressure, and 162 (1., about 95% of its content of aluminium oxide was dissolved. Working without the application the aluminium oxide. passed into solution,

The separation of impurities can be effected by other means. If' a solution of aluminium nitrate is prepared in the first instance and is subjected to the action of pressure, after removal of the insoluble residue and if desired after being concentrated,

The advantage of heating and raising. the

pressure in stages is that the. apparatus is not subjected for long to .thecorrosive action of the reagents, as. would otherwise'be the case.

A further method of removing iron I and other impurities is to treat the solution of aluminium nitrate with a concentrated solu-i tionof nitric acid or gaseousnitric acid. .337 treating with nitrate acid in thisway the all!- minium salt is precipitated, For exam 0, by adding double the quantity of concentra, ed nitric acld to a cold saturated solution of alu-.

minium nitrate, more than 80% of the minium nitrate .in solution may be prec par tated in the form of fine crystals. Even bet: ter results have been obtained hyusing gase- 1 ousnitric acid, After treating the aluminium nitrate solution in this way the impurities remain moved,

in solution and can be easilyre- .A high degree of purification, particularly with regard to iron, is thus obtained during the decomposition ortreatment of the alumine ium mtrate solution, and the last traces can be readily" and completely removed in the course of the process if the aluminium nitrate solution, after being concentrated, is treated with potassium ferro-cyanide in concentrated solution or in the solid state. If the 'solutionsarevery highly concentrated a precipitate of the term-cyanogen compoundis obtained, which can be easily filtered ofil For efiicient precipitation of the iron it is advantageous to apply heat, stir the mixture and keep the reaction neutral. 0

The terro -cyanogen compound obtained canbe used as a blue pigment, or it may be used for re-forming the original cyanogen compound, for example, by treatment with alkali, ammonia or alkali carbonate, In this way ferric hydroxide and acyanogen compound are obtained, the latter beingused again inthe process. Y j

Another method of removing the last traces of iron from the aluminium nitrate solution is to treat it'with a thiocyanate. By this treatment iron thiocyanate is formed which can be removed by means of ether. For recovering the residue the ether is evaporated, d h sidual iron thi cy nate s ea d with an (alcoholic) solution'of alkali, ani

monia oralkaline carbonate for recovering the thiocyanate. I, I

Even though cyanogen compounds, such as thiocyanogen compounds, have a relatively high market value, it is possible to use them economically in the process as there is no special difficulty in recovering them practically without loss. The solution of aluminium nitrate obtained by the extraction of the alnminiferous raw material, and partly or completely freed from iron, is used for the production of alumina. The reforming ofthe nitric acid from the alminium nitrate is important'and advantageous. Inorder'to carry out this recovery with a minimum of loss, the solution of aluminium nitrate is first highly concentrated by evaporation in vacuo, till the aluminium nitrate crystals are fused in their water of crystallization... This melt? is then more strongly heatedso that the oxides of nitrogen are evolved in a highly concentrated condition. It iS'suiEcient to work at relatively low temperatures, so that it is possible to-use the ordinary,apparatus with externaliapplication of heat. A rotary .riln may be used. i

The indicated temperature conditions are particularly advantageous in practice-tor the decomposition of aluminium nitrate. For example, temperatures between 300 and 500 C. are quite suitable, but with the operation carried out intwo stages, i. e. the removalof water and the decomposition, higher or lower temperatures may be used.

Instead of effecting "the removal of basic impurities during the decomposition or extraction, or in addition to doing this, solid aluminium nitrate or a paste of crystals thereof may be produced by evaporation or cooling. tageous in the case of asolution or". basic aluminium nitrate, produced by effecting the decomposition. with a deficiency of nitric acid. Theremoval of iron is particularly thorough if precautions are taken to bring the iron present in the reacting solution into the colloidal form. For example, this be etfected by. adding bases, suchas aluminium oxide, alkaline earths, or alkalies,.to the solu tion of decomposition products obtained, until the required degree of basicity is obtained. Alternatively the solution may be evaporated until sufficient nitric acid vapour has been driven elf. to produce the required degree of basicity. v

Another method of removing impurities is to evaporate the solution of aluminium ntrate and then to add concentrated or gase 'us nitric acid. I have already indicated that by adding concentrated nitric acid aluminium nitrate is precipitated. If the aluminium nit'rate solution is simultaneously evaporated, the greater part of the aluminium nitrate in This method is particularly advan-' the solution can be crystallized outat a temperature above 415 0., and in this way it obtained in a purer form.

flhe crystals of aluminium nitrate so obtained are generally washed with nitric acid. It is preferable to wash with concentrated nitric acid, as in this way impurities are removed. Purification is also eftected by treating solid aluminium nitrate or a paste of crystals thereof with gaseous'nitric acid.

The decomposition of aluminium nitrate may be efi'ected by heatingv it in the solid, crystallized, or dissolved state, or in the state in which it occurs after the last step in the process, if desired, with the introduction of gases or of water or steam. In this way nitric acid of any desiredconcentration can be produced.

'lt'is preferable to carry out the evaporation of the aluminium nitratesolution in the neutral gases (nitrogen, carbon dioxide), and

it is equally advantageous to effect the decomposition of the aluminium nitrate with the introduction of gases, whether the heating and decomposition are eli'e'cted inone operation or in two steps, and whether the action is carried out in vacuo, in partial yacuo or under pressure.

It has been found preferable to effect the decomposition of the aluminium nitrate vacuo. The decomposition intoaluminium oxide and nitric acid then ta lres place easily and steadily at relatively low, temperatures, and nitric acid of very highconcentration is readily produced.

In this way about 92% of the nitric acid in the aluminium nitrate can be recovered, therefore nitrous gases are formed only in smallquantity. Even at 1l (1,75% of the nitric acid may be distilled over, and the remainder at. about 200. These remarkably good results are obtainable at an absolute pressure of 220 mms. I

The preparation of alumina fromaluminium nitrate can be carried out by decomposition in vacuo, equally well whether the aluminiumnitrate is in crystalline or dissolved condition, or it free nitric acid is present during the decomposition. The degree of concentration of the nitric acid obtained in addition to aluminium oxide is of course dependent on the water present, or the water formed by decomposition. If aluminium nitrate containing very little water is to be decomposed, and it it is desired to produce nitric acid of a given concentration it is necproceeds'in such mannerthat as the nitric acid is distilled off aluminium nitrates of increasing basicity are formed. The basic aluminium nltrate, swells up when heated,

forming films and viscous, greasy. bodies,

which resist the COIIClIlCtlOIIOf heat- By add ing aluminato the decomposition nitrate this is avoided. For example,a substance containing a high percentage. of alumina may be added to the nitrate in the proportion of one part to two parts. The aluminamay, however, be added to the evaporated solution of aluminium nitrate, and the mixture decomposed. Another method is tl'ie'spraying of the 'evaporated'solution of aluminiuninitrate mixedwith alumina. a

, Another method .of decomposing the aluminium nitrate is to introduce the material to be decomposed into a suitable melt or into a liquid of suitable boiling point, if necj' 'essary with agitation, and to distill off the oxides of nitrogen at the required decomposition temperature and at atmospheric pressure, or under partial vacuum, with the mtroduction of steam, air, oxygen or gas.

The advantages of the introduction of steam or gases, are that the partial pressure f can be reduced and that more complete con- (Iii version of thedecomposing gases into nitric 7 acid is obtained. By carrying out the deposition in the melt proceeds un formly and completely, and much more uniformly than with other methods of decomposition. It is' P probable that the melt at least in some cases also acts as a catalyst.

It is a great-advantage that practically no nitrous gases are formed, as this would give rise to a loss of nitric acid, I i

. For'example, sodium nitrate may be used as a melt. As this salt melts at slightly above 300 C. it forms a suitable melt (heating bath), and enables the decomposition of the rapidly and aluminium nitrate to be a thoroughly eiiected in simple apparatus, such as a boiler having stirring means. When steamis introduced, or water added,'nitric.

acid and aluminium oxide result, the latter in heavy, granular form, which. after completion of the reaction and upon treating the melt with water, sinks to the bottom of the aqueous solut on. Thus the separation of the aluminium oxlde in the melt is s1mpl1fied,the

upper portion of the melt being first decanted used instead of the sodium nitrate. Calcium nitrate, may be used as a melt. 7

. nitrate may be evaporated down with alumin; 'ium nitrate, to be decomposed and used as 0,.

melt for effecting decomposition. A mixture. of sodium and potassium nitrates may be nitrate, in particular hydrated calcium lVhen the decomposition is carried out with the aid of a melt it is convenient to work v-icuo. It is also preferable to introduce es,tor example, steam, for driving out the iiitricacid. As already mentioned in connection the decomposition of aluminium nitrate in a m lt, the introduction of gases during heating operation is advantageous. a

The aluminium oxide obtained by the de-' composition may be further purified. It has been shown that any remaining iron com} pounds or-other impurities can be practically wholly removedbytreatment with concern trated nitric acid. aflternatively the aluminium oxide obtained by decomposing aluminium nitrate may be dissolved in soda lye, if

necessary with the application of heat in an autoclave. The sodium aluminate obtained is filtered and the aluminium separated by pre cipitati'on in known manner. i

For the extraction of the raw material, for

the

the production ofthe solutions of alum nium Q nitrate, dilute nitric acids 'can be used.

Therefore nitric acid used for washi 'f precipitated aluminium nitrate or' the allii mina, generally for purifying in course of process, can be used for-extracting or the heat treatment of the raw material. By 1 i evaporating the solution and by decomposition of the aluminium nitrate, highly concen-.

trated' or gaseous nitric acid can be reformed in a very simple manner and reused in the rocess. All steps of the process can take place in vessels made of acid-proof alloys, thus the: extraction or dissolving, the heat treatment, the decomposing-reaction etc.

Examples of. suitable acid-proof alloys, already mentioned above, are alloys of iron, chromium and nickel, alloys containing tungsten and the like, and refined steels. It'is remarkable that vessels made of such materials permit of treatment under pressure in spite of the presence of nitric acid. i

The process enables very pure alumina to be obtained in a very simple manner, and no reagent used need be lost in the form of a valuable but unrecoverable by-produet; both the extracting and purifying agents can be recovered in the course of the process, and are repeatedly used in the process. This applies particularly to the nitric acid used for extracting or for the heat treatment ofv the raw material, and to nitric acid used in the concentrated or gaseous form for removing iron from the aluminium nitrate solution, or from the solid aluminium nitrate, or from the crystalline pastethereoflor for washing the pre cipitated aluminium nitrate.

Emdmples 1. 100 kg. of clay calcinedat 500 C. are treated with 200 litres of nitric acid of 400 grams per litre strength for four hours at about 0., the mixture being stirred. The hot solution is then freed from insoluble residues by means of a suction filter. The filtrate contains about 140 kg. of AMNOQ QH O in solution, about 2.2 kg. Fe(NO 9I-I O and small quantities of alkaline earths and alkalies. All the silicic acid and the titanic acid together with the greater part of the iron remain in the residue. The solution is then evaporated in vacuo and subsequently heated to 500 until the nitrate is completely decomposed. The oxides of nitrogen evolved are collected and are used for a subsequent extraction. -There remains 19 kg. of A1 0 The resulting alumina, which contains about 0.45 kg. of Fe O is dissolved in soda lye with or without the application of pressure, and is further treated in known manner, for instance according to Bayers method. The loss of alkali is thus considerably less than by direct alkaline decomposition of bauxite or clay.

2. 100 kg. of crystals of aluminium nitrate, which are obtainedby concentrating the nitrate solution, preferably a solution containing nitric acid as'it occurs after the reaction, and which need not be separated from the 1 mother liquor and which contains about 2.2%

Fe O with reference to the A1 0 are twice washed in the cold with concentrated nitric acid (sp. gr.=1.a After this treatment the iron content falls to about 0.5% Fe O while only traces of aluminium nitrate pass into solution. The resulting nitric acid containing small quantities of iron can be used again for the purifying process or for extraction.

3. 490 kg. of calcined clay, having an alumina content of about 23% are treated with 1000 litres of nitric acid (400 gm. HNO per litre) for about four hours at 8 atmospheres pressure and at a temperature of about 160 C. Thealuminium'nitrate solution produced (containing about 100 kg. of A1 0 is practically pure, the iron content in relation to the content of A1 0 amounting to only 0.7%.

After separating the residue, the nitric acid used is recovered for a subsequent extraction during the thermal decomposition of the aluminium nitrate.

4.. The solution obtained by treating 525 kg. of calcined clay (containing about 23% iil O with 1000 litres of nitric acid (containing 400 gm. HNO per litre) at the ordinary pressure and at a temperature of about 80 C. is separated from the undissolved residue.

The solution of aluminium nitrate containing about 102 kg. of A1 0 contains iron tothe extent of about 2% Fe 'Og in relation to the quantity of A1 0 in solution. This .L solution after being" concentrated is subjected to pressure as set forth i -Example 2. 'A brown precipitate is formed. There is practicady no loss of aluminium nitrate from the'solution. The loss amounts to about 1'; to 2% of M 0 The quantity of iron in the solution drops toabout 0.6% of Fe O With reference to'the amount of A1 0 in the solution.

l'claim: I l

1. The method of producing pure alumina, which comprises treating aluminiferous material with nitric acid insufficient tocombine with all the alumina in thematerial, sepa rating the aluminum nitratesolution and dissociating the latter by heat.

2. In the method of producing pure alumina, treating aluminiferous material for several hours with nitric" acid insufficient to combine with all the alumina in the material and at a temperature of 80100 C.

3. In the method of producing pure alumina, mixing aluminiferous materialto a paste with nitric acid, allowing the mixture to stand at a temperature above atmospheric temperature, then adding additional nitric acid, the total acid being insufficient to combine with all the alumina in the material.

l. The method of producing alumina, which comprises treating aluminiferous material under heat and pressure with nitric acid insufficient to combine with all the alumina present, and decomposing the result- 1 ing aluminum nitrate into aluminav and acid.

5. In the method of producing pure alumina, mixing aluminiferous material and nitric acid insufiicient to combine with all the alumina of the material while subjecting it stepwise to increase of heat and pressure, and decomposing the resulting aluminum nitrate by heat.

6. In the process of producing pure alumina, treating aluminiferous material with '8. In the process of producing pure alu- Lu mina, treating aluminiferous material with nitric acid insufiicient to combine with all the alumina of the material, separating the solution of aluminum nitrate and deccmposing r it anaera 1 7 {nina, treating.alunii'ni erousmaterial with V nitric acid insuflicint, to 'combinewitli all the alumina of the material; separating the solulion of aluminum nitrate and decomposing under a pressure less than atmospheric a.

. pressurewhile heating it in nitric acid and 7 r iji ai iio fji s sulre e heating itin nitric aci 9. In ther p roc'ss of rod'ucingi'pure'alw gaseous nitric acid.

10. Iin the process of producing purealumina, treating aluminiferous material with nitric acid, insufficient to combine with all the alumina of themateriahseparating thes'olution of aluminum nitrate and decomposin r es'suie ess than atmos 'heri 

